Cellulose is a polysaccharide having a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units. Cellulose is the structural component of the primary cell wall of green plants, many forms of algae and the oomycetes. Cellulose is also the major component of wood, and thus of paper, and is the most common organic compound on Earth. About 33 percent of all plant matter is cellulose (the cellulose content of cotton is 90 percent and that of wood is 40-50 percent).
Cellulose Whiskers (CW) also known as nanocrystalline cellulose (NCC) are fibers produced from cellulose; the NCC are typically high-purity single crystals. They constitute a generic class of materials having mechanical strengths equivalent to the binding forces of adjacent atoms. The resultant highly ordered structure produces not only unusually high strengths but also significant changes in electrical, optical, magnetic, ferromagnetic, dielectric, conductive, and even superconductive properties. The tensile strength properties of NCC are far above those of the current high volume content reinforcements and allow the processing of the highest attainable composite strengths. A review of the literature on NCC, their properties, and their possible use as a reinforcing phase in nano-composite applications is given by [1-3].
Another type of nano-cellulosic material is nano-fibers, known as Micro Fibrilited Cellulose (MFC) or Nano fibrillated cellulose (NFC) which are produced, e.g., by enzymatic treatment of mainly bleached pulp followed by shearing and homogenization of mainly bleached pulp. In some cases enzymatic pretreatments are applied in order to reduce the required production energy. Due to the relatively mild conditions employed, the amorphous cellulose remains intact, resulting in micrometer long fibers with a nano-metric diameter [4].
Bacterial Cellulose (BC) is a nanostructured extracellular product obtained from certain cellulose producing bacteria such as Gluconobacter Xilinus [5]. The cellulose fibrils, being generally of higher crystallinity and purity than those obtained from plant sources (as no lignin or hemicelluloses are present), are inherently of nano-sized dimensions in their cross-section.
Polymeric foams are materials with high importance in the field of composite materials. Foams are used for many applications, e.g. for insulation, structural parts such as car dash boards, as well as for core materials in manufacturing of composite sandwich panels to achieve high strength, energy dissipation, insulation, and weight reduction. Conventional foams are produced from oil based polymers such as foamed polyvinyl chloride (PVC), polyethylene (PE), polyurethane (PU), polystyrene (PS), polymethacrylimide (PMI) and polypropylene (PP). Polymeric foams convey high insulation and weight reduction properties; however, some have low strength and as all are fossil oil-based, they impress a clear environmental disadvantage.
Recently, it was shown that NCC as well as nano-fibers can be processed into foams by simple methods. The preferred method for production of such foams is by casting NCC or nano-fibers suspension into molds followed by freeze-drying. The foams maybe produced also by any other foaming technique such as supercritical fluid extraction, micro-fluidics, etc. The resulting foams also termed aerogels are highly porous and light weight. Nevertheless, these foams display low resistance for compression and therefore their utilization as core materials is limited [3].
NCCs have been shown to significantly improve the mechanical properties of polymeric composite materials. However, to achieve homogeneous suspension of the NCC in a polymer resin, high energy and often expensive equipment is necessary [6]. Pranger and Tannenbaum [7] have demonstrated that furan resin can be dispensed by dry sulfur treated cellulose nano-fibriles and serve as catalyst for furan polymerization.